It is a real good bet that many people are going to go to the Beijing Olympics and they are not going to have their hotel reservations set up in advance, and are going to try and wing it. I think that it is an equally good bet that people that thought that they had reservations are going to find that they don't for one reason or another. Another real good bet is that the prices are going to be going higher than the pole vault for the last minute reservations, so what to do?
The north side of Beijing is going to be fully booked this is where the great majority of the sporting events are going to take place and it is where the Olympic village is located. There are universities in this area where they are sure to be putting people up. When scouting that area of Beijing last year, there were not many hotel rooms up in that area. One nice hotel that we found was the Oriental Bay hotel which is a Beijing 4 star. I would put lots of money that this hotel will be fully booked. The official Beijing Olympics hotel the Raffles - Beijing Hotel are also going to be fully booked this is around 1000 rooms for the one facility and it is not cheap at anytime being really 2 world class 5 star hotels combined with everything from bowling alleys to Jade carvings that take up whole walls to xylophone like music in the atrium, that soars to the heavens.
So where does one look to find good cheap hotels for the Olympics at the last minute. One of the great things about Beijing and the Olympics here is the subway system which will get you into most areas of the city. Beijing is setup in expanding squares with the Olympics mostly taking place in northern quadrants between the 3rd and 5th rings. The two areas where I would look would be south of the Forbidden city and the area around the Beijing Train Station. Then I would start looking on the long line that runs east and west and look further outside of the city with the best bet heading in the direction of the airport.
Home stays and the internet are going to be available and when the Chinese figure out how much they can make for their places they are going to be doubling up and visiting relatives to free up space. These spaces not being traditional guesthouses will be harder to find. One of the interesting things about China is that they love the internet and have huge computer rooms where they go and socialize. I am sure that there will be chartrooms that you will be able to find that will network you to the available rooms & accommodations.
Check the local push taxi guys they are very familiar with their little areas and will be good sources of information for you that is outside of the normal channels and with negotiation you can bet that these will be the cheapest accommodations available in Beijing.
Being an adventurer myself I would try this, have a note made in Chinese that says that you are looking for a room in a family home and then walk around in the Hutongs near the Forbidden City. If you are lucky and find one you will get a real look at Beijing where most others will only see the shiny patina.
